The present invention relates an LIFD valve assembly and to a valve block having many such LIFD valves.
The fundamental structure of such LIFD valve assemblies is known, for instance from European Patent Disclosure EP 0 566 449 A1 or EP 0 566 449 B1. This is a hydraulic control arrangement on the load-sensing principle, in which in each case an adjusting pump is set as a function of the highest load pressure of the actuated hydraulic consumer in such a way that the inflow pressure is above the highest load pressure by a defined pressure difference. The pressure medium flows to the hydraulic consumers via adjustable metering apertures, which are each located between a pump line, originating at the adjusting pump, and the respective consumer. By means of pressure balance devices connected downstream of the respective metering apertures, it is attained that, given a sufficient quantity of pressure medium furnished regardless of the load pressures of the hydraulic consumers, a certain pressure difference exists via the metering apertures, so that the quantity of pressure medium flowing to a hydraulic consumer now depends only on the opening cross section of the respective metering aperture. If one metering aperture is opened wider, then more pressure medium must flow by way of it in order to generate the defined pressure difference. The adjusting pump is adjusted in each case such that it furnishes the required quantity of pressure medium. This is therefore also known as demand flow control.
The pressure balance devices downstream of the metering apertures are urged in the opening direction by the pressure downstream of the respective metering aperture and in the closing direction by a control pressure, prevailing in a rear control chamber, that is typically equivalent to the highest load pressure of all the hydraulic consumers. If upon a simultaneous actuation of a plurality of hydraulic consumers the metering apertures are made to be so wide open that the quantity of pressure medium, furnished by the hydraulic pump adjusted up to a stop is less than the total quantity of pressure medium pumped, then the quantities of pressure medium flowing to the individual hydraulic consumers are reduced in proportion, regardless of the load pressure of the various hydraulic consumers. This is accordingly called control with load-independent flow distribution (LIFD control).
To prevent the load from collapsing if the pump pressure is inadequate, a load-maintaining device is located in the pressure medium flow path between each consumer and the pressure balance device associated with it. This is typically embodied with a valve cone, which upon a reverse flow of pressure medium from the consumer in the direction of the metering aperture blocks off the pressure medium flow path essentially without leakage, so that the consumer cannot collapse in the event of an unwanted reduction of the pump pressure. The disadvantage of this embodiment is that considerable engineering effort is needed to integrate the load-maintaining device or load-maintaining devices into the valve block. Moreover, these load-maintaining devices require complex conduit courses and occupy considerable installation space, so that a compact embodiment of the valve assembly is possible only with difficulty. A further disadvantage is that the load-maintaining devices have high hydraulic resistance.
To overcome this disadvantage, it is proposed in U.S. Pat. No. 5,535,663, European Patent Disclosure EP 1 023 508 B1, and U.S. Pat. No. 5,067,389 that the individual pressure balance devices associated with the respective consumer be embodied in two parts, with one upper part and one lower part, the lower part acting as a load-maintaining device. In all these known versions, the two-part pressure balance device can be manufactured only with major production effort and expense. Moreover, the conduit course for picking up the individual load pressure downstream of the metering aperture is embodied in a very complex fashion.